1. Field of the Invention
The present invention relates to composite material including a metal matrix reinforced by internal, generally parallel, continuous high-strength, high modulus carbon/graphite fibers integrally bonded within and to the matrix material.
2. Description of the Prior Art
Graphite and carbon fibers, because of their high strength, are recognized as having excellent potential for reinforcing metal matrix composites. However a lack of chemical compatibility between graphite and many metals and difficulties in achieving a strong mechanical bond in the fiber to metal interface has retarded the development of these composites. Yet another problem area is created by the physical nature of most of the presently commercially available fibers. The fibers are in multi-filament (tow and yarn) form consisting of 1000 to 100,000 strands each of a diameter of about seven microns (typical). Although many metals in liquid form wet the outer surfaces of the fibers, the metals will not penetrate or infiltrate and bond with the inner strands of the fibers. When there is no bond the full strength of the fibers is not imparted to the composite.
Aluminum alloys appear to offer the most potential as a matrix for graphite reinforced metals and consequently have commanded most of the research effort. The usage of an aluminum-graphite composite will be largely controlled by its cost. At a moderate price this composite will be competitive for many applications in aircraft, missiles, automotive, electrical machinery, rocket propulsion systems, launch vehicle structues, and spacecraft. In aircraft, such applications are skins, struts, spars, wing boxes, and helicopter blades. Important applications requiring the 1000.degree. F temperature capability of the aluminum-graphite composite are re-entry shielding for missiles, and compressor blading for gas turbine engines. Launch vehicle structures require light weight materials for stiffening large diameter cylindrical sections, interstages, adapters, tank and equipment support structures. Examples of applications in spacecraft are shells and trusses in the primary structures, and booms, solar cell panels, equipment mounts, and antennas in auxiliary structures. With low cost composites becoming available, widespread non-aerospace industry applications will arise for aluminum-graphite composites. Some such applications are in rapid transit, deep submergence vehicles, sporting goods and rotating parts in electrical generators.
Aside from structural applications graphite-reinforced metals such as copper, aluminum and lead have other unique properties of interest such as high strength combined with good electrical conductivity, low coefficient of friction and high wear resistance, or high dimensional stability over a range of temperature. Copper-graphite and aluminum-graphite composites are of interest for high strength electrical conductors, and aluminum-graphite, lead-graphite, and zinc-graphite composites have potential as bearing materials.
The most promising approach to achieving fiber penetration by and bonding with metals is by coating the fibers with a compatible lamina of submicron thickness. Typical of this approach are the composite preparation processes as described by Sara in U.S. Pat. Nos. 3,553,820 and 3,571,901. Another such process is found in our U.S. Pat. application Ser. No. 131,823 filed Apr. 6, 1971, now U.S. Pat. No. 3,770,488.
Generally, these prior processes have many meritorious features. However they do not provide an optimal solution since some are not well adapted to high volume production while in others dewetting and debonding of the fibers from the metal matrix is experienced in subsequent production processes. Still others fail to achieve a uniform and complete good quality bond between the metal matrix and its reinforcing fibers.